Internal combustion engines for heavy-duty and industrial applications are typically fuelled by diesel. However, the use of natural gas as an alternative to diesel is of increasing interest. Natural gas is relatively abundant and relatively cheap, and can, in principle, provide similar levels of power to diesel whilst producing lower particulate and nitrogen oxide (NOx) emissions.
Natural gas can be used in place of diesel to fuel a compression-ignition engine, in which combustion of the fuel occurs as a result of compression of the air-fuel mixture in the cylinder. However, because natural gas has a higher auto-ignition temperature than diesel, it can be necessary to initiate combustion with a pilot injection of diesel fuel before introducing the natural gas to the combustion chamber.
In one type of natural gas-powered engine, known as a high-pressure direct injection (HPDI) engine, both natural gas and diesel are injected directly into the combustion chamber. Due to the space constraints in an engine cylinder head, it is desirable to inject both fuels using one fuel injector per cylinder. This requires a fuel injector that is specially adapted to keep the two fuels separate within the injector, and to deliver independently the respective fuel at the appropriate time.
One such ‘dual fuel’ injector is described in International Patent Application Publication No. WO 00/15956. In this example, a fuel injector with a concentric twin nozzle arrangement is provided. Inner and outer valve needles are engageable at their lower ends with respective valve seats to control the flow of fuel through respective inner and outer sets of outlets. The outer valve needle controls the injection of natural gas through the outer set of outlets, and the inner valve needle controls the injection of diesel through the inner set of outlets. The outer valve needle is tubular to accommodate the inner valve needle, and the inner set of outlets is formed at a tip of the outer valve needle.
The inner and outer valve needles are controlled independently by two electromagnetic control valves, which are configured to control the pressure of a control fluid (normally diesel fuel) within respective control chambers for the inner and outer valve needles. The control chambers receive the upper ends of the respective needles, so that changing the pressure of the control fluid in each control chamber changes the downward (closing) force on the corresponding needle. Gas or diesel fuel pressure acts on downwardly-facing thrust surfaces of the respective needles to generate an upward (opening) force on the needle. When the pressure of the control fluid in a control chamber is relatively high, the downward force is greater than the upward force and the respective needle remains seated, and when the pressure of the control fluid is relatively low, the upward force overcomes the downward force and the respective needle opens to permit fuel injection through the respective set of outlets.
Each control chamber is connected to a source of control fluid at relatively high pressure. Each control valve is operable to connect the respective control chamber to a low-pressure drain for the control fluid. In this way, opening of each control valve causes a reduction in the pressure of the control fluid in the corresponding control chamber, resulting in opening of the corresponding valve needle.
The valve needles are typically housed in a nozzle body of the injector, with the fuel outlets disposed at a tip of the nozzle body. The nozzle body is attached to a nozzle holder or injector body by a cap nut. In use, the injector is mounted in a bore in the cylinder head of the engine. The injector extends through the bore, so that the tip of the nozzle body protrudes into the respective combustion chamber. The maximum diameter of the cylinder head bore, and hence the maximum diameters of the cap nut, the nozzle body and the injector body of the injector, are constrained by the limited space available in the cylinder head.
The restricted maximum diameter of the nozzle body can be particularly limiting in the design of dual fuel injectors. The nozzle body must accommodate supply passages to deliver the two fuels to the tip region for injection, and may also accommodate service passages to connect with one or both of the control chambers. Accordingly, it would be desirable to maximise the amount of space available within the nozzle body.
The nozzle body typically defines both the seating region and a needle guide region for the outer valve needle. To mitigate wear or other damage to the seating region due to the closing impact of the outer valve needle, the nozzle body is usually made from a high-strength steel, such as a tool steel. Tool steel is relatively expensive and difficult to machine, which makes the component cost of the nozzle body relatively high. It would therefore also be desirable to reduce the cost of the nozzle body.